Indiana Jones and the Kingdom of the Crystal Skull?

I think they said it someplace in the 3rd movie that you would have to keep drinking from the cup every day - one cup would not make you immortal forever - look at the 3 brothers (the knights), 2 had died, and only one was left to guard the cup. He told them that was the price you pay - if you want to live forever, the cup had to stay within the seal - that means drinking from it every day. If you left the area, you would stop drinking from the cup and die like normal people.